[Fortran] Another array reference dependency improvement

[Paul Thomas]

Roger,

>This latter approach avoids the need to pass down a linked list
>of variables through gfc_dependency's call stack, and then scan
>this list everytime we encounter a variable at a leaf.
>
>Perhaps the gfortran maintainers could indicate which approach
>they think is most appropriate.
>
>  
>
I favour the latter - ie. a context flag. In doing the Polyhedron tests 
to check out your first patch, I notice that compilation times are 50% 
up on 6 months ago; there's too much running up and down linked lists!

Paul